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He was born about 1200 in the diocese of Bath, and educated at Oxford (Greyfriars) under the famous Robert Grosseteste. Before 1226 Marsh received the benefice of Wearmouth from his uncle, Richard Marsh, Bishop of Durham; but between that year and 1230 he entered the Franciscan order. About 1238 he became the lecturer of the Franciscan house at Oxford, and within a few years was regarded by the English province of that order as an intellectual and spiritual leader. Roger Bacon, his pupil, speaks highly of his attainments in theology and mathematics.
His fame, however, rests upon the influence which he exercised over the statesmen of his day. Consulted as a friend by Robert Grosseteste, as a spiritual director by Simon de Montfort, the countess of Leicester and the queen, as an expert lawyer and theologian by the primate, Boniface of Savoy, he did much to guide the policy both of the opposition and of the court party in all matters affecting the interests of the Church. He shrank from office, and never became provincial minister of the English Franciscans, though constantly charged with responsible commissions. Henry III and Archbishop Boniface unsuccessfully endeavoured to secure for him the see of Ely in 1256. In 1257 Marsh's health was failing, and he appears to have died two years later.
To judge from his correspondence he took no interest in secular politics. He sympathized with Montfort as with a friend of the Church and an unjustly treated man; but on the eve of the baronial revolution he was on friendly terms with the king. Faithful to the traditions of his order, he made it his ambition to be a mediator. He rebuked both parties in the state for their shortcomings, but he did not break with either.
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Marsh, Adam". Encyclopædia Britannica. 17 (11th ed.). Cambridge University Press. p. 768.
- Herbermann, Charles, ed. (1913). Catholic Encyclopedia. New York: Robert Appleton Company. .